1. Field of the Invention
The present invention relates to an X-ray diagnostic apparatus that takes a shot by subjecting a patient to X-ray exposure.
2. Description of the Related Art
An imaging system includes an X-ray tube and an imaging device. Most of today's imaging devices include an image intensifier and a TV camera. An imaging device adopting a flat panel detector equipped with a solid-state image sensor is expected to come into widespread use in the near feature. The field of view of the imaging system is limited to the field of view of the imaging device.
There is a shoot technique to obtain an image of a range wider than the limited field of view of the imaging system. This shoot technique includes an intermittent movement shoot method in which the imaging system moves intermittently, and a continuous movement shoot method in which the imaging system moves continuously. In either shoot method, shots are repeated in association with the movement of the imaging system. A plurality of images generated through repetitive shots are jointed according to their respective shot positions. An image covering a range wider than the field of view of the imaging system is thus generated.
Stepping DSA (Digital Subtraction Angiography) is known as a typical example of the intermittent movement shoot method. Bolus DSA is known as a typical example of the continuous movement shoot method.
A contrast medium injected into a patient moves on the flow of blood. The imaging system is moved either intermittently or continuously to chase the moving contrast medium. As shown in FIG. 14, shots are repeated while the imaging system is kept moved.
A plurality of images generated through repetitive shots are subtracted from mask images at the respective corresponding shot positions. A plurality of subtraction images are then jointed according to their respective shot positions. A blood vessel extracted image corresponding to a wide range is thus generated.
According to the bolus DSA, a contrast medium of a high concentration is injected into a patient in an extremely short time. The contrast medium of a high concentration injected into the patient by the bolus injection method moves within blood vessels of the patient as a bolus. The imaging system repeats shots while chasing the contrast medium that is moving as a bolus.
The contrast medium injected into the patient starts to diffuse with time and is diluted little by little. A diluted contrast medium gives rise to deterioration in the ability to extract blood vessels. The bolus DSA method has an advantage in suppressing the deterioration of the blood vessel extracting ability.
According to the bolus DSA method, movements of the imaging system and shot triggers are often manipulated manually by a radiographer. The radiographer predicts the flow of blood and moves the imaging system to determine the timing of a shot trigger. The flow of blood differs from patient to patient and from region to region. Further, the flow of blood varies even in the same region of the same patient. Hence, not only a high degree of concentration and attention, but also considerably high skills are needed for the moving manipulation of the imaging system and the shot trigger manipulation.
The contrast medium does not necessarily flow in the blood vessels in the form of a perfect bolus even when it is injected by the bolus injection method. The contrast medium starts to diffuse little by little with time, and it cannot be avoided that a concentration of the contrast medium is lowered little by little with time due to diffusion. This, on rare occasion, causes an unwanted event that the contrast of the contrast medium becomes too low for use in diagnosis.